1. Technical Field
Example embodiment's of the present invention relates to the demodulation of received signals and, more specifically, to methods of demodulating received signals, which may be applied to a reception apparatus for receiving a signal having one or more signals mixed therein, and apparatuses for performing the method.
2. Description of the Related Art
A relay cooperative communication system using Complex Field Network Coding (CFNC) is advantageous in that it has more improved throughput than a relay cooperative communication system using Galois Field Network Coding (GFNC) or a relay cooperative communication system not using a network coding system because the number of time slots necessary for communication is small.
More particularly, the relay cooperative communication system using CFNC requires only two time slots for transmission and reception. In the first time slot, a Relay Station (RS) simultaneously receives signals transmitted from an eNB and User Equipment (UE), and in the second time slot, the RS simultaneously transmits a signal using CFNC to the eNB and the UE. In the first time slot, the two received signals cannot be separately processed because the signals transmitted from the eNB and the UE are simultaneously received by the RS. Accordingly, the RS uses a Maximum Likelihood Decision (MLD) method in order to distinguish the signals, overlapped with each other as described above, from the signals transmitted from the eNB and the UE, respectively.
In the case in which the RS decides the signals transmitted from the eNB and the UE using an M-ary modulation method, however, the relay cooperative communication system using CFNC has a disadvantage in that actual implementation is difficult because a computational load is sharply increased if a high-order modulation method is used because the MLD method having computational complexity of O(M2) is used.
The MLD method is a demodulation method theoretically having the best performance and is a method of taking combinations of pieces of information that may be transmitted from a transmission apparatus into consideration, applying the combinations to respective estimated channel characteristic matrices, and finding a combination of first transmission signals having the most similar result as received signals from among the results. Accordingly, the MLD method has not side effect, such as a noise rise, in terms of an information theory and shows better performance than other demodulation methods. The MLD method, however, has a disadvantage in that a computational load increases by geometrical progression according to an increase in the complexity of the modulation method. For example, if a transmission apparatus uses a modulation method of 64QAM, a combination of signals that may be transmitted by the transmission apparatus is 64×64. It is substantially difficult to review the combination of the signals in real time for each symbol period.
FIG. 1 is a conceptual diagram showing the operation of a conventional relay cooperative communication system using CFNC.
Referring to FIG. 1, the relay cooperative communication system using CFNC includes an eNB 10, an RS 20, and UE 30.
First, in a first time slot, the eNB 10 and the UE 30 transmit signals x1 and x2 to the RS 20 at the same time. The RS 20 that has received the signals from the eNB 10 and the UE 30 at the same time decides the signals transmitted from the eNB 10 and the UE 30 by using an MLD method because it cannot separately process the received mixed signal.
Next, the RS 20 generates one signal xRS from the decided signals by using CFNC and transmits the signal xRS, generated in a second time slot, to the eNB 10 and the UE 30.
Here, the eNB 10 and the UE 30 receive signals, such as those described in Equation 1 below.
                    {                                                                                                                        Y                      eNB                                        ⁡                                          [                      k                      ]                                                        =                                                                                                              H                                                      RS                            -                            eNB                                                                          ⁡                                                  [                          k                          ]                                                                    ⁢                                                                        X                          RS                                                ⁡                                                  [                          k                          ]                                                                                      +                                                                  N                        eNB                                            ⁡                                              [                        k                        ]                                                                                            ,                                                    eNB                                                                                                                                      Y                      UE                                        ⁡                                          [                      k                      ]                                                        =                                                                                                              H                                                      RS                            -                            UE                                                                          ⁡                                                  [                          k                          ]                                                                    ⁢                                                                        X                          RS                                                ⁡                                                  [                          k                          ]                                                                                      +                                                                  N                        UE                                            ⁡                                              [                        k                        ]                                                                                            ,                                                    UE                                                          Equation        ⁢                                  ⁢        1            
In Equation 1 above, YeNB[k] refers to the signal transmitted from the RS 20 and received by the eNB 10, HRS-eNB[k] refers to a Channel Frequency Response (CFR) between the RS 20 and the eNB 10, and NeNB[k] refers to the Additive White Gaussian Noise (AWGN) of the eNB 10. Furthermore, YUE[k] refers to the signal transmitted from the RS 20 and received by the UE 30, HRS-UE[k] refers to a CRF between the RS 20 and the UE 30, and NUE[k] refers to the AWGN of the UE 30. XRS[k] refers to the signal transmitted from the RS 20.
Next, the eNB estimates the signal, transmitted from the UE, by using the received signal and the signal transmitted by the eNB in accordance with a Maximum Likelihood Decision (MLD) method, such as those listed in Equation 2 below, and the UE estimates the signal transmitted from the eNB by using the received signal and the signal transmitted by the eNB.
                    {                                                                                                                                                X                        ^                                            UE                                        ⁡                                          [                      k                      ]                                                        =                                                            X                                              UE                        ,                        i                                                              =                                                                  min                                                                              X                                                          UE                              ,                              i                                                                                ∈                          A                                                                    ⁢                                              (                                                                                                                                                                                                                                                                                                                                          Y                                          eNB                                                                                ⁡                                                                                  [                                          k                                          ]                                                                                                                    -                                                                                                                                                                                                                                                                                                                                                                                H                                                                                          RS                                              -                                              eNB                                                                                                                                ⁡                                                                                      [                                            k                                            ]                                                                                                                          ⁢                                                                                                                              X                                            eNB                                                                                    ⁡                                                                                      [                                            k                                            ]                                                                                                                                                              -                                                                                                                                                                                                                                                                                                                                                                                    H                                                                          RS                                      -                                      eNB                                                                                                        ⁡                                                                      [                                    k                                    ]                                                                                                  ⁢                                                                  X                                                                      UE                                    ,                                    i                                                                                                                                                                                                      )                                                                                            ,                eNB                                                                                                                                                                            X                        ^                                            eNB                                        ⁡                                          [                      k                      ]                                                        =                                                            X                                              eNB                        ,                        i                                                              =                                                                  min                                                                              X                                                          eNB                              ,                              i                                                                                ∈                          A                                                                    ⁢                                              (                                                                                                                                                                                                                                                                                                                                          Y                                          UE                                                                                ⁡                                                                                  [                                          k                                          ]                                                                                                                    -                                                                                                                                                                                                                                                                                                                                                                                H                                                                                          RS                                              -                                              UE                                                                                                                                ⁡                                                                                      [                                            k                                            ]                                                                                                                          ⁢                                                                                                                              X                                            UE                                                                                    ⁡                                                                                      [                                            k                                            ]                                                                                                                                                              -                                                                                                                                                                                                                                                                                                                                                                                    H                                                                          RS                                      -                                      UE                                                                                                        ⁡                                                                      [                                    k                                    ]                                                                                                  ⁢                                                                  X                                                                      eNB                                    ,                                    i                                                                                                                                                                                                      )                                                                                            ,                UE                                                                        Equation        ⁢                                  ⁢        2            
In Equation 2 above, A refers to M signals according to an M-ary modulation method. Furthermore, XeNB[k] refers to the signal transmitted from the eNB 10, XUE[k] refers to the signal transmitted from the UE 30, and {circumflex over (X)}eNB[k] and {circumflex over (X)}UE[k] refer to the respective estimated values of the signals transmitted from the eNB 10 and the UE 30.
FIG. 2 is a block diagram showing the construction of the RS using a conventional MLD method.
Referring to FIG. 2, the reception antenna 21 of the RS 20 simultaneously receives signals transmitted from the eNB 10 and the UE 30. Here, the signal received by the reception antenna 21 is a signal, such as that listed in Equation 3.yRS[n]=heNB-RS[n]*xeNB[n]+hUE-RS[n]*xUE[n]+nRS[n]  Equation 3
In Equation 3, ySs[n] refers to the signal received by the RS 20, heNb-RS[n] refers to a Channel Impulse Response (CIR) between the eNB 10 and the RS 20, and xeNB[n] refers to the signal transmitted from the eNB 10. Furthermore, hUE-RS[n] refers to a CIR between the UE 30 and the RS 20, nRS[n] refers to the AWGN of the RS 30, and xUE[n] refers to the signal transmitted from the UE 30.
A guard interval removal unit 22 removes the guard interval of the signal received by the reception antenna 21 as in Equation 3. An FFT unit 23 transforms the signal from which the guard interval has been removed into a signal of a frequency domain through a Fast Fourier Transform (FFT) process.
Next, an MLD unit 24 performs MLC for the signal transformed into the frequency domain as described above. In general, the RS 20 can estimate a CIR and a CFR, and the MLD unit 24 performing MLD for the estimated CFR includes an operation unit for calculating a correlation value as in Equation 4 and a comparison unit for determining a minimum value.
                              {                                                                      X                  ^                                eNB                            ⁡                              [                k                ]                                      ,                                                            X                  ^                                UE                            ⁡                              [                k                ]                                              }                =                              min                                          X                                  eNB                  ,                  i                                            ,                                                X                                      UE                    ,                    i                                                  ∈                A                                              ⁢                                                                                                        Y                    RS                                    ⁡                                      [                    k                    ]                                                  -                                                                            H                                              eNB                        -                        RS                                                              ⁡                                          [                      k                      ]                                                        ⁢                                      X                                          eNB                      ,                      i                                                                      -                                                                            H                                              UE                        -                        RS                                                              ⁡                                          [                      k                      ]                                                        ⁢                                      X                                          UE                      ,                      i                                                                                                          2                                              Equation        ⁢                                  ⁢        4            
In Equation 4 above, A refers to M signals according to an M-ary modulation method. Furthermore, YRS[k] refers to the signals received by the RS 20, HeNB-RS[k] refers to a CFR between the eNB 10 and the RS 20. and HUE-RS[k] refers to a CFR between the UE 30 and the RS 20. Furthermore, {circumflex over (X)}eNB[k] and {circumflex over (X)}UE[k] refer to the respective estimated values of the signals transmitted from the eNB 10 and the UE 30.
A Complex Field Network Coding (CFNC) unit 25 calculates a complex addition signal from the two signals (i.e., {circumflex over (X)}eNB[k] and {circumflex over (X)}UE[k]), decided using Equation 4, by using Equation 5.XRS[k]=WeNB[k]·{circumflex over (X)}eNB[k]+WUE[k]·{circumflex over (X)}UE[k]  Equation 5
In Equation 5, XRS[k] refers to the signal transmitted from the RS 20, and WeNB[k] and WUE[k] refer to weights (e.g., phase change values) applicable in various ways.
An Inverse Fast Fourier Transform (IFFT) unit 26 performs an IFFT for the signal (i.e., XRS[k]) generated in accordance with Equation 5 above, and a guard interval insertion unit 27 inserts guard intervals into the signal subjected to the IFFT. Furthermore, a transmission antenna 28 transmits the signal, provided from the guard interval insertion unit 27, to the eNB 10 and the UE 30.
FIG. 3 is a constellation diagram showing 16QAM signal pairs which are used in the conventional MLD method.
Referring to FIG. 3, Xi indicates 16 signals which are all the 16QAM signals transmitted from an XeNB, and Xj indicates 16 signals which are 16QAM signals of an XUE.
As shown in FIG. 3, since the number of {Xi, Xj} signal pairs is 162, the conventional MLD method of calculating all the signal pairs and then deciding the XeNB and the XUE by calculating a minimum value requires a total of 256 calculation processes. Alternatively, if an eNB and UE use 64QAM, the conventional MLD method requires a total of 4096 calculation processes because the number of {Xi, Xj} signal pairs is 642. Accordingly, a computational load sharply increases.
A demodulation method using the conventional MLD method as described above has a disadvantage in that actual implementation is difficult because a computational load increases in geometrical progression as a high-order modulation method is used.